Folded cascode differential stages are useful for single-supply, low voltage, integrated circuit operational amplifiers for several reasons, including the ability to handle a wide common-mode input voltage range, the capability to provide both voltage translation and differential to single-ended current conversion, operation over a wide range of supply voltages, and ease of implementation in an integrated circuit.
In prior art operational amplifiers the bias current levels are determined by a current source I.sub.s connected with the input stage and a bias voltage VB connected with a differential to single ended output stage. However, care must be taken in the generation of I.sub.s because of the direct effect variations in I.sub.s have on the current I2 in the output stage. If the current I.sub.s is too large, I2 may be reduced to zero. Conversely, if the current I.sub.s is too low in value, the differential current in the input stage may not be large enough to be significant when compared to mismatch currents in the output stage. These conditions put upper and lower bounds on I.sub.s relative to I2 and a good choice for the value of I.sub.s is equal to the desired value of I2. The current source I.sub.s is typically established with a "Widlar" current source including resistors which makes it difficult to maintain a precise relationship between I.sub.s and I2.